The resurgence of fetal gene products as a concomitant, if not the causative event, of biochemical dedifferentiation of tumors will be studied systematically in order to put existing observations to that effect on a firmer and broader footing. Solid, transplanted tumors of various organs of the rat and cells cultured in vitro, before and after viral transformation, will be used for this purpose. The multiple molecular forms of given enzyme activities (isoenzymes) will be separated by electrophoresis, localized by histochemical staining on the electrophoretograms, and used as indicators of altered macromolecular biosynthetic activity of neoplastic tissues. The appearance of fetal isozymes will be temporally and quantitatively correlated with other manifestations of malignant transformation, particularly with the tumorigenic potential of transformed cells, in the in vitro system. Agents known to promote enzymic differentiation during normal development will be tested for their possible ability to reverse biochemical dedifferentiation in neoplasias.